Wednesday, March 17, 2010

A newly discovered star outside the Milky Way has yielded important clues about the evolution of our galaxy. Located in the dwarf galaxy Sculptor some 280,000 light-years away, the star has a chemical make-up similar to the Milky Way’s oldest stars, supporting theories that our galaxy grew by absorbing dwarf galaxies and other galactic building blocks.

Some recent studies had questioned the link between dwarf galaxies and the Milky Way, citing differences between the chemistry of their stars. But the differences may not be so big after all, according to new research published in Nature. “It was a question of finding the right kind of star, and doing that required some new techniques,” says Josh Simon , an astronomer at the Observatories of the Carnegie Institution and a member of team that confirmed the star’s telltale chemistry. Using earlier techniques, he says, “it was very difficult to recognize exactly which stars were the key ones to study.”

Dwarf galaxies are small galaxies with just a few million stars at most. They often orbit larger galaxies such as the Milky Way, which consists of hundreds of billions of stars. In the “bottom-up model” of galaxy formation, proposed in 1978 by Carnegie astronomers Leonard Searle and Robert Zinn (now at Yale University), large galaxies attained their size over billions of years by swallowing up their smaller neighbors. But if dwarf galaxies are the building blocks of larger galaxies, then the same kinds of stars should be found in both types of galaxies, especially in the case of old, “metal-poor” stars. (To astronomers, “metals” are chemical elements heavier than hydrogen and helium.) Because they are products of stellar evolution, metals were rare in the early Universe, and so old stars tend to be metal-poor.

Stars in the Milky Way’s halo can be extremely metal-poor, with metal abundances as much as 100,000 times less than the Sun, which is an average metal-rich star. Surveys over the past decade had failed to turn up any such extremely metal-poor stars in dwarf galaxies. “The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies,” says Simon. “If dwarf galaxies were the original components of the Milky Way, then it’s hard to understand why they wouldn’t have similar stars.”

Simon and his colleagues suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused the surveys to miss the most metal-poor stars. Team member Evan Kirby, a Caltech astronomer, developed a method to estimate the metal abundances of large numbers of stars at a time, making it possible to search efficiently for the most metal-poor stars in dwarf galaxies. Among stars he found in the Sculptor dwarf galaxy was one designated S1020549. Spectroscopic measurements of the star’s light at Carnegie’s Magellan-Clay telescope in Las Campanas, Chile, determined it to have a metal abundance more than 4,000 times lower than that of the Sun―five times lower than any other star found so far in a dwarf galaxy.

“This star is likely almost as old as the universe itself,” said astronomer Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, lead author of the Nature paper reporting the finding.

In addition to the star’s total metal abundance, researchers also compared the abundance of iron to that of elements such as magnesium, calcium, and titanium. The ratios resembled those of old Milky Way stars, lending more support to the idea that these stars originally formed in dwarf galaxies.

The researchers expect that further searches will discover additional metal-poor stars in dwarf galaxies, although the distance and faintness of the stars pose a challenge for current optical telescopes. The next generation of extremely large optical telescopes, such as Carnegie’s proposed 24.5-meter Giant Magellan Telescope, equipped with high-resolution spectrographs, will open up a new window for studying the growth of galaxies through the chemistries of their stars.

In the meantime, says Simon, the unusually low metal abundance in S1020549 study marks a significant step towards understanding how our galaxy was assembled. “The original idea that the halo of the Milky Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct.”

Cornell researchers have found a protein that may lead to a new way to control mosquitoes that spread dengue fever, yellow fever and other diseases when they feed on humans: Prevent them from urinating as they feed on blood.

The work may lead to the development of new insecticides to disrupt the mosquito's renal system, which contributes to a mosquito's survival after feeding on blood.

Aedes aegypti mosquitoes transmit the virus that causes dengue fever, putting 40 percent of the world's population at risk of catching the disease, and causing 50 million to 100 million infections (22,000 deaths) annually. They pick up diseases when feeding on infected hosts and can then infect new hosts when they feed again. Currently, no vaccine or treatment protects against dengue, so the only way to stop its spread is by controlling mosquitoes.

But now, a Cornell study published in the March 4 issue of the American Journal of Physiology -- Regulatory, Integrative and Comparative Physiology has identified a protein from the renal tubules of Aedes aegypti mosquitoes that appears to be involved in promoting urination as they feed on blood. When mosquitoes consume and process blood meals, they must urinate to prevent fluid and salt overloads that can kill them.

Also, "they have to undergo rapid urination when feeding, or they can't fly away," said Peter Piermarini, the paper's lead author and a postdoctoral research associate in the lab of Klaus Beyenbach, a professor of biomedical sciences in Cornell's College of Veterinary Medicine and the paper's senior author. "Too much weight will impair the mosquito's flight performance, like an aircraft with too much payload. [If they get too heavy,] they may become more susceptible to being swatted by their host or eaten by a predator," said Piermarini.

The researchers discovered a key protein expressed in the mosquito's renal system that contributes to urination. In lab experiments, Piermarini, Beyenbach and colleagues demonstrated that blocking the protein's function in the renal tubules with a drug reverses the enhanced rates of urination that would occur during blood feeding.

"Thus, blocking the function of this protein in natural populations of mosquitoes may limit their ability to survive the physiological stresses of a blood meal and to further transmit viruses," said Piermarini.

The Aedes aegypti renal system also serves as a valuable model for parts of the mammalian kidney, with similar cells in each system and possibly similar proteins, said the authors.

A study published in the American Chemical Society's journal Nano Letters reveals that thermocells based on carbon nanotube electrodes might eventually be used for generating electrical energy from heat discarded by chemical plants, automobiles and solar cell farms.

The research was a joint collaboration between Baratunde Cola, assistant professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech, and an international team of researchers from the U.S., Australia, China, India and the Philippines.

Cola, director of Georgia Tech’s NanoEngineered Systems and Transport Research Group (NEST), described the study as a breakthrough in efficiently harvesting electrical energy from various sources of exhaust or wasted heat.

"Our NEST Lab was fortunate to team with Dr. Ray Baughman's NanoTech Institute at UT Dallas and Dr. Gordon Wallace's Intelligent Polymer Research Institute in Wollongong, Australia, in the final year of a long collaboration that solved key technical problems,” he said. “We brought fresh eyes, as well as our knowledge and experience with heat transfer engineering from the nanoscale to the scale of practical devices to the problem, which provided a key missing link. The team will together work to enable additional breakthroughs that are required for this technology to reach its full commercial potential."

Efficiently harvesting the thermal energy currently wasted in industrial plants or along pipelines could also create local sources of clean energy that in turn could be used to lower costs and shrink an organization’s energy footprint.

The new thermocells use nanotube electrodes that provide a threefold increase in energy conversion efficiency over conventional electrodes.

One of the demonstrated thermocells looks just like the button cell batteries used in watches, calculators and other small electronics. One key difference, however, is that these new thermocells can continuously generate electricity, instead of running down like a battery. The research netted other thermocells, as well, including electrolyte-filled, textile-separated nanotube sheets that can be wrapped around pipes carrying hot waste streams from manufacturing or electrical power plants. The temperature difference between the pipe and its surroundings produces an electrochemical potential difference between the carbon nanotube sheets, which thermocells utilize to generate electricity.

The research team estimates that multi-walled carbon nanotubes in large thermocells could eventually produce power at a cost of about $2.76 per watt from freely available waste energy, compared with a cost of $4.31 per watt for solar cells, which can only be used when the sun is shining. On a smaller scale, button cell-sized thermocells could be used to power sensors or electronic circuits.

The new thermocells take advantage of the exceptional electronic, mechanical, thermal and chemical properties of carbon nanotubes. The nanotubes’ giant surface area and unique electronic structure afforded by their small diameter and nearly one-dimensional structure offer high current densities, which enhance the output of electrical power and the efficiency of energy harvesting.

"Georgians have worked with state support, and in partnership with initiatives such as the Strategic Energy Institute at Georgia Tech, to realize significant gains in renewable energy production,” Cola said. “But to become a leading energy state, we must increasingly explore new ways to extract and utilize all forms of energy. Harvesting waste heat as electricity is one direction our NEST Lab takes with international partners to help provide increased renewable energy options for Georgia and the world."

Scientists in Manchester have found a clean and green way of making tiny magnets for high tech gadgets – using natural bacteria that have been around for millions of years.

The work by a team of geomicrobiologists from the University of Manchester paves the way for nanometer-size magnets – used in mobile phones and recording devices – to be made without the usual nasty chemicals and energy intensive methods.

Researchers studied iron-reducing bacteria that occur naturally in soils and sediments and found they can be used to create iron oxide nanoparticles with magnetic properties similar to those created through complex chemical processes.

Working with colleagues in Birmingham and Cardiff, the Manchester researchers also found a way of exercising precise control over the size and magnetic strength of nanomagnets produced.

The high-tech particle accelerators at the Advanced Light Source at the famous Berkeley Labs near San Francisco, and the UK’s Diamond Light Source in Oxford at Harwell were used to verify findings.

Researchers added cobalt, manganese or nickel to the basic iron-based energy source used by bacteria, which resulted in the production of tiny magnets containing these elements. This greatly enhanced their useful magnetic properties.

Aside from being used in the latest gadgets, nanomagnets also have the potential to be used in drug delivery systems and cancer therapies to carefully focus and target the release of chemicals into the body.

Metal-reducing bacteria live in environments deficient in oxygen and react with oxidised metals to produce natural magnets in the ground beneath our feet.

And now the research team has developed a way of harnessing pure strains of these bacteria – which are in plentiful supply and reproduce quickly – to produce large quantities of nanomagnets at an ambient temperature.

This compares favourably to the extreme temperatures – as high as 1000 degrees Celsius – needed to create nanomagnets using current methods.

Prof Richard Patrick, Professor of Earth Science, said: “This is exciting work that raises the exciting prospect of a biologically friendly, energy-efficient method of producing nanomagnets tailored for different uses.”

A paper – ‘Harnessing the extracellular bacterial production of nanoscale cobalt ferrite with exploitable magnetic properties’ – outlining the research was published recently in the journal ACS Nano.

Astronomers from the United States and Europe have used a gravitational lens -- a distant, light-bending clump of dark matter -- to make a new estimate of the Hubble constant, which determines the size and age of the universe.A paper describing the work appears in the March issue of The Astrophysical Journal.

The Hubble constant has previously been calculated by using NASA's Hubble Space Telescope to look at distant supernovae, and by measurements of the cosmic microwave background -- radiation leftover from the Big Bang, said Chris Fassnacht, associate professor of physics at UC Davis. The new method provides an independent check on the other two, he said.

A gravitational lens is a distant object, such as a galaxy surrounded by dark matter, that exerts a gravitational pull on light passing through it. Other galaxies behind the lens, from our point of view, appear distorted. In the case of the object B1608+656, astronomers on Earth see four distorted images of the same background object.

Fassnacht began studying B1608+656 as a graduate student a decade ago. Because the mass distribution of the lens is now well understood as a result of recent Hubble Space Telescope observations, it is possible to use it to calculate the Hubble constant, he said.

It works something like this. Two photons of light leave the background galaxy at the same time and travel around the lens, their paths distorted in different ways by the gravitational field so that they arrive on Earth at slightly different times. Based on that time delay, it is possible to calculate the distance of the entire route, and then infer the Hubble constant.

The timing is set by waiting for a change in the background object -- for example, for it to become more luminous. If the travel times are slightly different, the different images of the background object will seem to brighten at slightly different times. Imagine two drivers leaving Stanford to drive to Davis, one by the East Bay and one through San Francisco, Fassnacht said. Assuming both drivers maintain the exact same speed, they will arrive at Davis at different times. That difference can be used to work out the overall distance.

Gravitational lensing has never before been used in such a precise way, said co-author Philip Marshall of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at the U.S. Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. Several groups are now working on extending the technique with other gravitational lenses.

The next time a buzzing bug dodges the swipe of your swatter, consider that these household pests are among nature's most acrobatic and well-adapted flying machines. Using high-speed cameras and computer models, Cornell researchers have shown exactly how fruit flies maneuver through the air, and how they keep stable even when a whoosh of wind knocks them off course.

Led by Jane Wang, professor of mechanical and aerospace engineering; Itai Cohen, assistant professor of physics; and John Guckenheimer, professor of mathematics, the researchers invented a method to measure the subtle flapping wing motions that drive in-flight turning maneuvers. They found that flies use a mechanism familiar to anyone who has rowed a boat: Their wings slice through the air as they flap one way, and then push off the air as they sweep back. By pushing harder with one wing than the other, the insects can do a U-turn in 0.1 seconds -- fast enough to avoid the swatter.

This paper, with first author Attila Bergou, Ph.D. '09, is posted online at http://arxiv.org/abs/0910.0671 and is under review at Physical Review Letters.

In a related project, the research team also revealed the secret behind the stability of flying insects. When the flight of a fly is disturbed, such as by a sharp gust of wind, the insect uses an automatic stabilizer reflex that keeps it upright and on course. This reflex tells the insect exactly how hard to paddle its wings so it can recover from a midflight "stumble" faster than the blink of an eye.

These observations were published online March 1 in Proceedings of the National Academy of Sciences in a paper whose first author is graduate student Leif Ristroph.

Cohen's group, which studies the physics of soft condensed matter, is "fascinated by flight," Cohen said, and that interest is not just fancy-free fun.

"The ability to control flight profoundly changed the evolution of our planet," Cohen said. About 350 million years ago, the world was devoid of the many trees and flowers of today. Once insects figured out how to maneuver through the air, these pollinators enabled the Earth to blossom with life.

The research on insect flight, Cohen said, could also simplify the design of maneuverable and stable flapping-wing aircraft.

The team made its observations with three high-speed video cameras that recorded every slight motion of the insects as they responded to patterns of light and dark stripes. This optical illusion tricked the bugs into performing in-flight turns.

The researchers found that the insects paddled their wings to steer while flying, delicately adjusting the inclination of their wings by miniscule amounts -- as little as 9 degrees -- at a remarkable rate of 250 times a second.

To unlock the secrets of flight stability in these flies, the researchers devised a way to trip them up in midflight. They glued tiny magnets to the backs of flies sedated by a dunk into ice water. When the insect came to and flew around, the researchers turned on magnetic fields that zapped the magnet, nudging the insect off its flight path.

The insects took the disturbance in stride, quickly paddling their wings so that they could recover their original posture with pinpoint accuracy. All this is possible because of two small, vibrating sense organs called halteres, which millions of years ago evolved from what used to be a pair of hind wings. Mathematical models show how the halteres ultimately tell the wings to paddle. The fruit fly's quick reflex is similar to the first autopilots that kept airplanes from dropping out of the sky.